MEXICO CITY – U.S. and Mexican officials signed an agreement Wednesday allowing each country’s trucks to traverse the other’s highways, implementing a key provision of the 1994 North American Free Trade Agreement after nearly two decades of bickering.

Transportation secretaries Ray LaHood and Dionisio Perez-Jacome signed the three-year memorandum, which is based on an agreement announced in March by Presidents Barack Obama and Felipe Calderon.

NAFTA, signed in 1994, had called for Mexican trucks to have unrestricted access to highways in border states by 1995 and full access to all U.S. highways by January 2000. Canadian trucks have no limits on where they can go.

But until now, Mexican trucks have seldom been allowed farther than a buffer zone on the U.S. side of the border. In retaliation, Mexico had imposed higher tariffs on dozens of U.S. products.

The Mexican government has now agreed to suspend those tariffs as long as the agreement is in place.

The public debate surrounding the accord had mostly focused on the safety of Mexican trucks. But labor unions and other groups were strongly opposed to the agreement, which they say will cost Americans trucking and other jobs.

The U.S. Department of Transportation says the safety concerns have now been resolved. Electronic monitoring systems will track how many hours the trucks are in service. Drivers will also have to pass safety reviews, drug tests and assessments of their English-language and U.S. traffic sign-reading skills. Mexico has the authority to demand the same of U.S. truck drivers entering their territory.

But those won’t do much to resolve the U.S. debate over the migration of jobs, which dates back to the NAFTA debates of the early 1990s. The question: Will a freer flow of cross-border cargo traffic boost business and allow owners to hire more workers, or will it ship U.S. jobs to Mexican drivers who work for lower pay?

LaHood argued the first position Wednesday in an email to The Associated Press.

“By opening the door to long-haul trucking between the United States and Mexico, America’s third largest trading partner, we will create jobs and opportunity for our people and support economic development in both nations,” he said.

The Teamsters Union was incensed. General President Jim Hoffa said the agreement was “probably illegal” because it goes further than a previously agreed-on pilot program and described it as “opening the border to dangerous trucks at a time of high unemployment and rampant drug violence.”

Other U.S. groups from the National Cattlemen’s Beef Association to the National Christmas Tree Association celebrated the end of the punitive tariffs and hoped for higher sales. The tariffs tax $2.4 billion worth of U.S. exports according to the U.S. Chamber of Commerce, including tariffs up to 45 percent on certain fruits according to a trade group.

Those tariffs will be cut in half within 10 days and then eliminated completely when full cross-border traffic begins.